1. Field of the Invention
The present invention relates to an infrared ray detecting sensor.
2. Description of the Related Art
Infrared ray detecting sensors are used in a variety of fields, for example, infrared ray detecting sensors are mounted in automobiles or aircraft for determining a target at a distance or in darkness. The infrared ray detecting sensor includes therein an infrared ray detecting element formed of a binary or ternary compound semiconductor, which is usually used in a cold environment at a temperature of, for example, liquid nitrogen (77.degree. K) and accommodated in a heat insulating vacuum housing of a Dewar structure. The housing of a Dewar structure comprises axially extending and concentric inner and outer cylindrical members to form a chamber therebetween; the infrared ray detecting element being located in the chamber and resting on the inner end wall of the inner cylindrical member. An infrared ray permeable window is arranged in the end wall of the outer cylindrical member opposite the inner end wall of the inner cylindrical member, outside infrared rays passing through the infrared ray permeable window to the infrared ray detecting element. A cooler is arranged in a cavity inside the inner cylindrical member and the infrared ray detecting element is connected to an outside electrical component via leads and ceramic wiring boards.
The chamber formed in the housing is evacuated to a vacuum level and a getter is arranged in the chamber to maintain the chamber at a vacuum level. The getter comprises an enclosure of porous material capable of absorbing gas, as is well known, and an electric heater is arranged in the enclosure. The electric heater is activated during periodic services of the infrared ray detecting sensor.
In such an infrared ray detecting sensor, the getter and the ceramic wiring board, radiate to generate an infrared ray or infrared rays that may reach the infrared ray detecting element as a straying rays and creates a noise at the infrared ray detecting element. When the infrared ray detecting sensor is used in a stationary environment, the straying ray generated at the getter will not necessarily result in a noise since the detected straying ray is at a constant level and can be compensated by the control of the system. However, when the infrared ray detecting sensor is subjected to vibration, the straying ray reaching the infrared ray detecting element continuously changes and cannot be compensated by the control of the system. Thus, there is the problem that the sensitivity of the infrared ray detecting sensor and the quality of the target image thereby obtained is lowered. Also, when the infrared ray detecting sensor is subjected to vibration, contaminated particles in the chamber or particles removed from the getter create a noise at the infrared ray detecting element.
It is known to arrange a cylindrical cold shield on the inner end wall of the inner cylindrical member so as to surround the infrared ray detecting element (for example, refer to Japanese Unexamined Patent Publication (Kokai) No. 63-61123). In this reference, the cold shield extends axially from the inner end wall of the inner cylindrical member toward the infrared ray permeable window of the outer cylindrical member to define an aperture, and a disk or flange is attached to the free end of the cylindrical cold shield.
This disk can prevent a straying ray generated by the radiation of the outer cylindrical member from affecting the infrared ray detecting element and thus may solve the above described problem to some extent. However, the cold shield is attached to the inner end wall of the inner cylindrical member where the temperature of the housing is the lowest and does not extend close to the infrared ray permeable window, and the disk attached to the cold shield also does not extend close to the outer cylindrical member. Therefore, there is a sufficient gap between the disk and the outer cylindrical member to allow a straying ray and particles to reach the infrared ray detecting element. Also, there may be an infrared ray detecting sensor that does not include a cold shield. In addition, while this reference notes a straying ray generated by the radiation of the outer cylindrical member that is at room temperature, the straying ray generated at the getter and the ceramic wiring board is stronger than that generated at the outer cylindrical member and there is no attempt made to effectively prevent the straying ray generated at the getter from affecting the infrared ray detecting element.